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"Theorem 4 Let $\mathscr E$ be an equivalence relation on a nonempty set $X$. Then $X/\mathscr E$ is a partition of $X$.

[Proof] By Theorem 3(a) and Definition 6, $X/\mathscr E =\{\,x/\mathscr E\mid x \in X\,\}$ is a family of nonempty subsets of $X$. We next show that

$x/\mathscr E \neq y/\mathscr E ⇒ x/\mathscr E \cap y/\mathscr E = \emptyset$

by showing its contrapositive: $\color{red}{x/\mathscr E \cap y/\mathscr E \neq \emptyset \Rightarrow x/\mathscr E=y/\mathscr E}$.

The last assertion is a direct consequence of Theorem 3(b) and (c). Finally, we have to show that $\bigcup\limits_{x\in X} x/ \mathscr E = X$. This is also trivial, since each $x$ in $X$ belongs to $x/\mathscr E$."

The author assumes the partition $X/\mathscr E$ of $X$ satisfies two conditions to be a partition as the following:

(a) $(x/\mathscr E, y/\mathscr E \in X/\mathscr E) \land (x/\mathscr E \neq y/\mathscr E) \Rightarrow x/\mathscr E \cap y/\mathscr E = \emptyset$

(b) $\bigcup \limits_{x \in X}x/\mathscr E = X$

But as shown in the definition
$X/\mathscr E =\{\,x/\mathscr E\mid x \in X\,\}$,
$x/\mathscr E$ is the only element of $X/\mathscr E$, not $y/\mathscr E \in X$.

So why the author assumes $y/\mathscr E \in X$ is also an element of $X/\mathscr E$?

FYI

"Definition 5 Let $X$ be a nonempty set. By a partician $P$ of $X$ we mean a set of nonempty subsets of $X$ such that:

(a) If $A, B\in P$ and $A\neq B$, then $A\cap B=\emptyset$

(b) $\bigcup \limits_{C \in P}C = X$"

"Definition 6. Let $\mathscr E$ be an equivalence relation on a nonempty set $X$. For each $x∈X$, we define
​$$x/\mathscr E=\{\,y\in X\mid y\mathscr Ex\,\}$$
which is called the equivalence class determined by the element $x$.
The set of all such equivalence classes on $X$ is denoted by $X/\mathscr E$; that is, $X/\mathscr E=\{\,x/\mathscr E\mid x\in X\,\}$.
The symbol $X/\mathscr E$ is read "$X$ modulo $\mathscr E$," or simply "$X$ mod $\mathscr E$".

Source: Set Theory by You-Feng. Lin and Shwu-Yeng T. Lin

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The notation $\{\,x/\mathscr E\mid x\in X\,\}$ is the set of all objects of the form $x/\mathscr E$ that can be obtained by letting $x$ run over the elements of $X$. For example, if $X=\{1,2,3\}$ then $\{\,x/\mathscr E\mid x\in X\,\}=\{1/\mathscr E,2/\mathscr E,3/\mathscr E\}$ (which may possibly be simplified, for example if it happens to be the case that $1/\mathscr E=2/\mathscr E$, then this is in fact the set $\{1/\mathscr E,3/\mathscr E\}$). At any rate, in the given context we have that $y\in X$. Therefore $y/\mathscr E$ is an element of $\{\,x/\mathscr E\mid x\in X\,\}$


Formal definition: If $\Phi$ is a class function and $A$ is a set, then the Axiom Scheme of Replacement states that there exists a set $B$ with the property that $$\forall z\colon z\in B\leftrightarrow\exists x\in A\colon z=\Phi(x) $$ In other words, $B$ contains precisely what can be onbtained by applying $\Phi$ to elements of $A$. The set $B$ guaranteed to exist this way is also written as $\{\,\Phi(x)\mid x\in A\,\}$, where $x$ is a free variable, or sometimes as $\Phi[A]$.

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    $\begingroup$ Fun fact: this is a mathematical example of "confusing the map with the territory" $\endgroup$ Jan 29, 2016 at 16:14

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